Fix Issue With Location of xilinx_dma.h

[bperez77]

The purpose file version_portability.h is to deal with the differences between Linux 3.x and 4.x. This is to deal with changes to the generic DMA interface, and changes to Xilinx's AXI DMA driver. One of these changes is the location of xilinx_dma.h.

In 3.x versions, it was located under include/linux/amba/xilinx_dma.h and in 4.x it was typically located under include/linux/dma/xilinx_dma.h. However, not all 4.x versions of the Xilinx kernel obey this. For example, the Analog Device's fork of the kernel has the file under include/linux/amba/xilinx_dma.h. This was originally spawned by discussion in issue #1. The branch xilinx-v2015.4 of the mainline Xilinx kernel has this problem as well.

Thus, to deal with this quirk, the file has to updated to allow for a new macro that specifies to use the alternate location for the 4.x version. This should be able to be specified from the Makefile command line.